May 2013

Different parts of your visual brain areas process different parts of the visual world. The mapping between the brain and what we see around us is well understood and easy to map in anyone’s brain.

In order to make this map, we present a revolving or expanding stimulus on a computer monitor while subject’s brains are imaged with a magnetic resonance imaging (MRI) scanner. The MRI scanner measures both the physical structure of the subjects brain, as well as the the blood flow to different brain areas, or the Blood Oxygen Level-Dependent (BOLD) signal.

By keeping the revolutions or expansions constant and slow, one can measure in the BOLD signal voxels in the visual cortex where the blood flow follows the same revolution frequency as the stimulus. Each time the stimulus gets to the RECEPTIVE FIELD of the voxel, the neurons fire in response, and blood follows after a small lag. Voxels with different receptive fields will fire at the same FREQUENCY, but with a different PHASE at this frequency, (a different time in the cycle).

The maps below show the PHASE, or where in the cycle the voxel peaks. This phase maps onto stimuli at a certain location, which are shown in the coloured circular legends. Revolutions and expansions in opposite directions are averaged in order to cancel out the time lag of the blood to get to the brain.

You can also flatten out these maps in order to make a precise map of the visual space (shown in the legend circle). The black lines show separations between each of the different visual brain areas (V1, V2, V3..), which can be easily determined by the red and purple bands in the BOLD signal phase maps.

Here the information from the two maps are combined to show each brain areas map of the visual space show the separations between visual areas

Thanks very much to Dr. Paige Scalf at Arizona and her student Laura Cacciamani for help with this work, and also to an excellent bunch of online resources that help with parts of the steps needed. (SamPenDu; DougNGreve; XuCui; FreesurferWiki). Note that not one of these protocols actually works by itself, and they need to be combined in certain ways to work. More to come…

We can use the light we measure to make maps of brain activity. To do this we have to map each light source we measure into specific areas of the brain.

The path that light takes to get to a single detector from one of the sources is modeled based on understanding of the diffusion and scattering of light in the head. It takes the shape that we like to call a BANANA.

If I plot all of the bananas for one subject in one of our experiments, we get an interesting looking map of that subjects brain, or at least the parts that our light is able to shine on: Here are four of the banana brains: